General speaking, a conventional vertebral locking and retrieving screw is defective in design in that it is devoid of a specific angle capable of negotiating with the specific curvature of a vertebra. With a view to overcoming the defect of such a conventional bone screw as mentioned above, the inventor of the present invention discloses a variety of angled screws for use in locking and retrieving various vertebrae of the human spinal column, as exemplified in the U.S. Pat. Nos. 5,176,679; 5,196,014; 5,257,994 and 5,330,474. These variously angled screws disclosed by this inventor of the present invention are constructed on the basis of the fact that the curvatures of various vertebrae of the human vertebral column differ from one another. However, in a surgical operation, two bone screws having respectively a specific angle are never fastened onto the same vertebra after the vertebra in question has been fixed horizontally. This is due to the fact that the traditional surgical operation requires that the vertebra under treatmnet must be first fixed longitudinally and then fixed horizontally by means of an auxiliary fixing device. The concept of fastening two bone screws, each of which has a specific angle, onto the same vertebra began to take shape only after a vertebral locking and retrieving system having a center rod was disclosed by this inventor of the present invention in U.S. Pat. No. 5,387,212. In other words, a horizontal fixation and a longitudinal fixation of a deformed vertebra are attained by means of two bone screws which have respectively a specific angle and are fastened onto the deformed vertebra in such a manner that the two bone screws in question are not parallel to each other. With a view to reinforcing the fixation of the vertebra under treatment, a fixation crossbar is used. Such fixation crossbar must have a specific curvature so that the fixation crossbar can cooperate well with the two angled bone screws. The technical problem that is derived from the above-mentioned U.S. Pat. No. 5,387,212 is that the fixation crossbar must have a specific curvature.